Hidekazu Goto

First-principles molecular-dynamics calculations and STM observations of dissociative adsorption of Cl2 and F2 on Si(001) surface

We have studied the dissociative adsorption process of halogen molecules (Cl2 and F2) on the Si(0 0 1) surface by first-principles molecular-dynamics (FPMD) calculations and scanning tunneling microscopy (STM) observations. From FPMD calculations, we demonstrate that Cl2 and F2 molecules adsorb dissociatively at dangling bonds of a buckled dimer with no energy barrier, so that the buckled dimer becomes geometrically flat. In addition, STM observations show that the dissociative adsorptions of Cl2 and F2 induce buckled dimers at the SA step on the Si(0 0 1)-(2×1) surface to become symmetric dimers, in good agreement with the results of FPMD calculations.

Electronic structures of peanut-shaped fullerene tubes

Abstract We have investigated electronic structures of the peanut-shaped polymerized fullerene with so-called P55, P56 and P66 tubular linkage structures. The stable atomic configurations in these one-dimensional or two-dimensionally bundled tubes are searched out by a tight-binding calculation, and then their electronic structures are evaluated by using an ab initio density functional calculation. The electronic structures change drastically depending on the bonding interaction between the tubes. The P66 tube can show metallic conduction when it is bundled two-dimensionally.

First-principles study of electronic structure of deformed carbon nanotubes

Abstract On the basis of density functional theory, we study the electronic structures of five types of carbon nanotubes: the non-deformed (6,6) tube, the uniformly stretched tube along the tube axis, the uniformly compressed tube, the partially stretched tube and the partially compressed tube. The electron charge density increases at the compressed C–C bond of the partially stretched tube, while the density decreases at the stretched C–C bond of the partially stretched tube. In addition, the a1 and e1 states of the (6,6) tube contribute to the bonding along the tube axis and the a2 and e2 states are the bonds connecting the atoms in the same layers. Thus, the energy bands of the a1 and e1 states are sensitively affected by the deformation of the tubes along the tube axis.

Total-energy minimization of few-body electron systems in the real-space finite-difference scheme

A practical and high-accuracy computation method to search for ground states of few-electron systems is presented on the basis of the real-space finite-difference scheme. A linear combination of Slater determinants is employed as a many-electron wavefunction, and the total-energy functional is described in terms of overlap integrals of one-electron orbitals without the constraints of orthogonality and normalization. In order to execute a direct energy minimization process of the energy functional, the steepest-descent method is used. For accurate descriptions of integrals which include bare-Coulomb potentials of ions, the time-saving double-grid technique is introduced. As an example of the present method, calculations for the ground state of the hydrogen molecule are demonstrated. An adiabatic potential curve is illustrated, and the accessibility and accuracy of the present method are discussed.

A path-integration calculation method based on the real-space finite-difference scheme.

We propose a new path-integration calculation method to treat the time evolution of a wavefunction within the framework of the real-space finite-difference formalism, and also develop an effective scheme to compute the scattering wavefunction for an incident electron with arbitrary energy, in which an impulse wavefunction is adopted as an initial state of the time evolution. In this method, once the time evolution of the initial impulse wavefunction is calculated, all of the solutions in the scattering problem can be derived by means of Fourier analysis of the time-evolved wavefunction, which leads to a reduction of the calculation time. In order to test the applicability of our newly developed simulation procedures, we implemented simulations for the one-dimensional scattering problem. Each simulation showed the usefulness of the present scheme by yielding the steady scattering states in agreement with exact ones.

Electrochemical Etching Using Surface-Sulfonated Electrodes in Ultrapure Water

In conventional electrochemical etching, contamination of the etched surface with electrolytes is unavoidable. Electrolyte-free electrochemical etching will be applicable to electronic device manufacturing and precision nanoscale processing as a low-cost and environmentally friendly process. We propose a new electrochemical etching method that requires no electrolytes, but rather ultrapure water. In principal, a metal surface can be etched by OH - ions. Thus, we prepared a sulfonated cathode that increases the OH - ion concentration and carried out electrolysis in ultrapure water and Cu etching. The resultant electrolysis current was 8.9-fold higher than that obtained using unmodified electrodes. Furthermore, nearly 100% etching efficiency was achieved in etching a Cu surface in combination with the sulfonated cathode through the anodic reaction Cu + 2 OH - → Cu(OH) 2 . The resultant etched surface had no pits; its roughness was 4.3 nm Ra.

Essentially exact ground-state calculations by superpositions of nonorthogonal Slater determinants

An essentially exact ground-state calculation algorithm for few-electron systems based on superposition of nonorthogonal Slater determinants (SDs) is described, and its convergence properties to ground states are examined. A linear combination of SDs is adopted as many-electron wave functions, and all one-electron wave functions are updated by employing linearly independent multiple correction vectors on the basis of the variational principle. The improvement of the convergence performance to the ground state given by the multi-direction search is shown through comparisons with the conventional steepest descent method. The accuracy and applicability of the proposed scheme are also demonstrated by calculations of the potential energy curves of few-electron molecular systems, compared with the conventional quantum chemistry calculation techniques.

Fabrication of damascene Cu wirings using solid acidic catalyst

Abstract The copper damascene process is one of the most promising technologies for fabricating Cu wirings for electronic devices such as LSIs. In this research, the fabrication of damascene Cu wirings was conducted using solid acidic catalyst. When a Cu-plated wafer, whose oxide is a basic oxide is dipped into a mixture of oxidizing solution and acidic solution, surface atoms are ionized and etched off into the solution. However, because conventional nonelectrolytic etching does not have a reference surface, it is difficult to utilize for planarization. Therefore, a new nonelectrolytic machining method using a cation-exchange fabric instead of an acidic solution was developed. To be more precise, the planarization of a Cu-plated wafer was carried out by rubbing with the cation-exchange fabric in ozone water. Basically, this method exploits chemical reactions so that the physical properties of the workpiece surface are not deteriorated. Furthermore, this method uses no chemicals except for ozone water, which easily dissociates into water and oxygen molecules; thus, this method is a low-cost, environmentally friendly process. In this paper, as a preliminary experiment, the nonelectrolytic etching of a Cu sample using solutions of O3 and CO2 was carried out to inspect the dependence of the etching rate on [O3] and [H+]. The results indicate that the etching rate increased as [O3] and [H+] increased. When [H+] was high relative to [O3], a smooth etch-pit-free surface was achieved. Next, nonelectrolytic etching using a cation-exchange fabric was carried out, and properties similar to those in the case of etching using solutions were obtained. Finally, damascene Cu wirings were fabricated using ozone water and a cation-exchange catalyst.

Development of Eco-Friendly Electrochemical Etching Process of Silicon on Cathode

A process for etching a silicon single-crystalline surface using an ultrafine particle dispersion is proposed. The ultrafine particles contain quaternary ammonium hydroxide groups as ion-exchange groups, giving an alkaline dispersion. In the etching process, the fine particles and impurity ions can be easily separated by filtration or dialysis. After repeated dialyses, impurities in the dispersion, which include heavy metal ions and alkali metal ions known to result in a rough-etched surface and to affect the electronic properties of the semiconductor, were decreased easily. The method is suitable for application to the surface planarization of silicon single crystals and to the manufacture of semiconductor devices. Using the proposed alkaline etching dispersion, a Si(001) surface was successfully patterned by cathodic electrochemical etching at room temperature. In the patterning of p-type Si, the average etching rate was 0.42 nm/min, and the center line average roughness (Ra) of the etched surface was 0.173 nm for an area of 64 X 48 μm.

Ultra high vacuum compatible metal ion beam surface modification system

A high current ion system for surface modification has been developed. It consists of a liquid metal ion source, a focusing lens system, deflection plates, a mass separator, and a deceleration electrode system. It can produce a high purity metal ion beam with an energy in the range of 10 eV to 10 keV and ion current up to 20 μA in a vacuum as low as 10−9 Torr.